The invention relates to a papermachine fabric in the form of a spiral link belt covered with a non-woven fabric sheet.
Papermachine fabrics in the form of spiral link belts are disclosed in German Offenlegungsschrift No. 2,419,751 and U.S. Pat. No. 4,346,138. Such spiral link belts consist of a multiplicity of meshing synthetic resin helices. The windings of adjacent synthetic resin helices overlap to such an extent that a channel is formed. A pintle wire is inserted into said channel and secures the helices together. Such spiral link belts serve as conveyor and filter belts and recently have replaced the conventional drier felts in the drying section of papermaking machines. The use in paper manufacture has hitherto been confined predominantly to the drying section, because in this section of the papermaking machine the requirements as to marking characteristics, mechanical abrasion, and resistance to compression are minimal. However, it is a disadvantage of the spiral belts that the large voids in the interior of the spiral links carry along a large volume of air and, at high speeds of the papermaking machines, act as blowers. At the deflection points of the belts the entrained air is ejected and may cause the paper sheet to flutter and, at very high speeds, even to tear.
In the modern Uni-Run belt guide systems in which a single belt guides the paper sheet over all the rotary driers of the upper and lower cylinder rows the entrained volumes of air produce superatmospheric pressure in the wedge shaped spaces between the drier cylinder and the arriving belt which at high speed, forces said air through the drier fabric, and the air then blows the paper web from the supporting drier fabric and, in extreme cases, even tears it. For this reason, low air permeability is required for Uni-Run belts.
Attempts have been made to reduce the permeability of the spiral link drier belts. According to U.S. Pat. No. 4,362,776, the voids in the helices are filled with filler material in the form of monofilamentary, multifilamentary, or ribbon-shaped yarn. Similar methods are known from German Offenlegungsschrift No. 3,135,140 and French Patent Publication No. 2,494,318, where a fiber nap can be on the surface of the strips of filler material. According to U.S. Pat. No. 4,381,612, stuffing filaments of thermoplastic material are used which are caused to flow under the influence of heat and to expand and thereby fill the interior of the helices. However, this effects only partial filling of the helix interiors. The higher the degree of filling, the more difficult it is to introduce the filler material. According to this method the air permeability is reduced from a cfm value of 1000-1100 for an unfilled spiral link belt to values of about 150-200 cfm.
German Offenlegungsschrift No. 3,147,115 discloses a papermachine fabric composed of a spiral link belt covered with a sheet of non-woven fabric. The non-woven fabric is fastened to one side of the spiral link belt by means of a binder warp. Such papermachine fabrics are contemplated for use in the sheet forming section of a papermaking machine.
In some types of fine paper the coarse structure of the spiral link belts leaves pronounced marks, particularly if the spiral link belts are used in the first drier groups of the papermaking machine where the paper sheet is still rather moist, and thus soft and sensitive to marking. Therefore, drier fabrics for use in this section are provided with a covering in the form of a non-woven fabric made of fine fibers which is needled onto the finely woven drier fabric on specially designed machines. In this process numerous needles with barb-like ends penetrate the non-woven fabric and the supporting fabric at closely spaced points. The fibers of the non-woven fabric are entrained by the needles and are anchored in the supporting fabric.
Drier fabrics have been partially replaced by spiral link belts, and attempts have also been made to needle a non-woven fabric thereto in order to reduce marking and to obtain a closed, smooth surface which is less prone to carry along air. Due to the relatively large diameter of the synthetic resin wires from which the helices of the spiral link belts are wound, however, the needles damaged said wires. Such damage to the synthetic resin wires not only reduced the strength of the spiral link belt but also had the result that the damaged wires became prone to premature attack by hydrolysis.
The use of fluid jets in the manufacture of non-woven fabrics, per se, is disclosed in U.S. Pat. No. 3,485,706, and is known as the spun-lace technique.